Traffic flow control allows safe and efficient travel for motorists. At a typical automotive intersection, motorists traveling in opposing directions are given alternating rights-of-way via a set of standardized traffic signal beacons. Each beacon consists of a recognizable combination of green, yellow, and/or red electric signal lamps enclosed in a standard housing. These beacons face the desired directions of travel and are controlled from a common point with one traffic signal controller. All lamps are electrically home-run to the controller using 120VAC or other high-voltage AC power. The controller selects which lamps to illuminate at any given time while a conflict monitor prevents unsafe combinations of lamp illumination. The lamps themselves may be incandescent with filters or one of a variety of LED styles. Common patterns include round balls, arrows, and “Xs.” A UPS including a rectifier, battery, and inverter may be included at critical locations.
Another application of traffic flow control is a bridge collapse motorist warning system. While bridges are generally safe, they can fail. When they do, frequently motorists that were not even on the bridge at the time of collapse drive over the edge. This is because by the time motorists become aware of the hazard, they may no longer have adequate stopping distance. Bridge curvature can limit visibility of a hazard even under otherwise ideal visibility conditions. To limit this unnecessary loss of life and property, a series of flashing red traffic signal beacons may be spaced along the length of the bridge and driven by 120VAC line power, as further described in Mercier, J. J. and Marshall, R. A., “Bridge Collapse Detection and Motorist Warning System,” IEEE ITS newsletter Vol 7, No 3, September 2005. There is one important drawback to this system. It is possible to short the beacon power to ground or to water, tripping a circuit breaker and causing all the bridge's beacons to go dark at the only time they are needed. While there is a method of mechanical disconnection of the damaged cable section which may reduce this risk, it does not guarantee critical operation.
A bridge collapse motorist warning system must be activated by a bridge collapse detection system, a means to detect the failure of the bridge. Frequently, a cable is run the entire length of the bridge with a break in the cable indicating a structural failure. U.S. Pat. No. 6,972,687, Marshall, et al., “System and Method for Detecting A Structure Failure” illustrates such a system using a fiber optic cable sensor. Unfortunately, fiber optic cable is difficult to grip and attach to fixed points on a bridge. Also, optical fibers and high voltage power are run in separate conduits or cables, which are both very costly and difficult to install.